Diabetic retinopathy, age-related macular degeneration and glaucoma are the typical ocular diseases in which vascular disorders and impaired circulation have been observed. For example, an increasing body of evidence suggests that dysfunction of ocular microcirculation in the optic nerve influences the progression of glaucoma. Quantification of ocular circulation is therefore important for the diagnosis of ophthalmic diseases.
Currently, a number of methods have been used for measuring ocular perfusion. Fluorescein angiography (FA) provides useful qualitative information in health and disease; however, it only shows the superficial retinal vessels and does not assess deep perfusion, such as the microcirculation in optic nerve head (ONH), choroidal blood flow. Additionally, injection of dye can cause nausea and anaphylaxis, making it unsuitable as a tool for routine glaucoma assessment. Both laser Doppler flowmetry [e.g. Heidelberg Retinal Flowmeter (HRF)], which samples capillary flow over a small retinal area, and laser speckle flowgraphy, which provides a spot sample of blood velocity, can show differences between diseases and normal groups. However the measurements provided by these methods are too variable for diagnostic application due to dependence on the signal strength and the location of the small sampled area. Magnetic resonance imaging (MRI) has been proposed to quantitatively image ONH perfusion; however, the major limiting factor with this method is the small size of the ONH and limited resolution to detect focal or mild circulatory insufficiency.
Optical coherence tomography (OCT) is an imaging technique that has been widely used for diagnosis and management of ocular diseases. As a coherence detection technique, OCT can detect the Doppler frequency shift of the backscattered light that provides information on blood flow. Doppler OCT has been used for measuring total human retinal blood flow (TRBF) in patients with glaucoma, optic neuropathy, and diabetic retinopathy. With this method, global blood flow from central retinal vessels can be quantified, but local microcirculation cannot be resolved because the velocity range is too low for accurate Doppler measurement. To measure local microcirculation, we recently developed the split-spectrum amplitude-decorrelation angiography (SSADA) algorithm that provides high quality three-dimensional (3D) angiography using ultrahigh speed OCT. Because SSADA is based on the variation of reflectance in resolution cells of isotropic dimensions, it is equally sensitive to transverse and axial movements. Thus it may be able to provide more impartial estimates of local microvascular perfusion that are independent of beam incidence angle. In contrast, both Doppler and phase-variance OCT angiography are more sensitive to axial flow than to transverse flow. Therefore SSADA may be a good basis for quantitative angiography of the microcirculation within different vascular beds.
In vivo three-dimensional mapping of biologic tissue and vasculature is difficult due to the highly-scattering and absorptive nature of biologic tissue. Some current methods have slow scanning speeds making in vivo three-dimensional imaging difficult. Some other techniques having faster scanning speeds are still lacking due to their inability to scan deeply into biologic tissue without producing overlapped images, requiring the use of invasive procedures to scan the tissue of interest. Many techniques aimed at deeper imaging generally cannot provide deep imaging of tissue having moving material (e.g., blood flow). Therefore, methods to effectively image structure and/or tissue movement, such as blood flow, are of substantial clinical importance.
Optical coherence tomography (OCT) is an imaging modality for high-resolution, depth-resolved cross-sectional, and 3-dimensional (3D) imaging of biological tissue. Among its many applications, ocular imaging in particular has found widespread clinical use. In the last decade, due to the development of light source and detection techniques, Fourier-domain OCT, including spectral (spectrometer-based) OCT and swept-source OCT, have demonstrated superior performance in terms of sensitivity and imaging speed over those of time-domain OCT systems. The high-speed of Fourier-domain OCT has made it easier to image not only structure, but also blood flow. This functional extension was first demonstrated by Doppler OCT which images blood flow by evaluating phase differences between adjacent A-line scans. Although Doppler OCT is able to image and measure blood flow in larger blood vessels, it has difficulty distinguishing the slow flow in small blood vessels from biological motion in extravascular tissue. In the imaging of retinal blood vessels, Doppler OCT faces the additional constraint that most vessels are nearly perpendicular to the OCT beam, and therefore the detectability of the Doppler shift signal depends critically on the beam incident angle. Thus, other techniques that do not depend on beam incidence angle are particularly attractive for retinal and choroidal angiography.
Several OCT-based techniques have been successfully developed to image microvascular networks in human eyes in vivo. One example is optical microangiography (OMAG), which can resolve the fine vasculature in both retinal and choroid layers. OMAG works by using a modified Hilbert transform to separate the scattering signals from static and moving scatters. By applying the OMAG algorithm along the slow scanning axis, high sensitivity imaging of capillary flow can be achieved. However, the high-sensitivity of OMAG requires precise removal of bulk-motion by resolving the Doppler phase shift. Thus, it is susceptible to artifacts from system or biological phase instability. Other related methods such as phase variance and Doppler variance have been developed to detect small phase variations from microvascular flow. These methods do not require non-perpendicular beam incidence and can detect both transverse and axial flow. They have also been successful in visualizing retinal and choroidal microvascular networks. However, these phase-based methods also require very precise removal of background Doppler phase shifts due to the axial movement of bulk tissue. Artifacts can also be introduced by phase noise in the OCT system and transverse tissue motion, and these also need to be removed.
To date, most of the aforementioned approaches have been based on spectral OCT, which provides high phase stability to evaluate phase shifts or differentiates the phase contrast resulting from blood flow. Compared with spectral OCT, swept-source OCT introduces another source of phase variation from the cycle-to-cycle tuning and timing variabilities. This makes phase-based angiography noisier. To use phase-based angiography methods on swept-source OCT, more complex approaches to reduce system phase noise are required. On the other hand, swept-source OCT offers several advantages over spectral OCT, such as longer imaging range, less depth-dependent signal roll-off, and less motion-induced signal loss due to fringe washout. Thus an angiography method that does not depend on phase stability may be the best choice to fully exploit the advantages of swept-source OCT. In this context, amplitude-based OCT signal analysis may be advantageous for ophthalmic microvascular imaging.
One difficulty associated with OCT's application in microvascular imaging comes from the prevalent existence of speckle in OCT images obtained from in vivo or in situ biological samples. Speckle is the result of the coherent summation of light waves with random path lengths and it is often considered as a noise source which degrades the quality of OCT images. Various methods have been developed to reduce speckle in spatial domain, such as angle compounding, spectral compounding, and strain compounding. Speckle adds to “salt-and-pepper-like” noise to OCT images and induces random modulation to interferometric spectra which can significantly reduce contrast.
In spite of being a noise source, speckle also carries information. Speckle patterns form due to the coherent superposition of random phasors. As a result of speckle, the OCT signal becomes random in an area that is macroscopically uniform. If a sample under imaging is static, the speckle pattern is temporally stationary. However, when photons are backscattered by moving particles, such as red blood cells in flowing blood, the formed speckle pattern will change rapidly over time. Speckle decorrelation has long been used in ultrasound imaging and in laser speckle technique to detect optical scattering from moving particles such as red blood cells. This phenomenon is also clearly exhibited by real-time OCT reflectance images. The scattering pattern of blood flow varies rapidly over time. This is caused by the fact that the flow stream drives randomly distributed blood cells through the imaging volume (voxel), resulting in decorrelation of the received backscattered signals that are a function of scatterer displacement over time. The contrast between the decorrelation of blood flow and static tissue may be used to extract flow signals for angiography.
The speckle phenomenon has been used in speckle variance OCT for the visualization of microvasculature. Speckle patterns at areas with flowing blood have a large temporal variation, which can be quantified by inter-frame speckle variance. This technique termed “speckle variance” has been used with swept-source OCT demonstrating a significant improvement in capillary detection in tumors by calculation of the variance of the OCT signal intensity. A key advantage of the speckle variance method is that it does not suffer from phase noise artifacts and does not require complex phase correction methods. Correlation mapping is another amplitude-based method that has also recently demonstrated swept-source OCT mapping of animal cerebral and human cutaneous microcirculation in vivo. These amplitude-based angiography methods are well suited to swept-source OCT and offer valuable alternatives to the phase-based methods. However, such methods still suffer from bulk-motion noise in the axial dimension where OCT resolution is very high. Therefore, an amplitude-based swept-source angiography method that is able to reduce bulk-motion noise without significant sacrifice in the flow signal would be optimal. For example, imaging of retinal and choroidal flow could be particularly improved with such noise reduction, as in the ocular fundus the flow signal is predominantly in the transverse rather than axial dimension.
While improving qualitative blood flow measurement through noise reduction methods has immense value, determining quantitative blood flow measurement in regions of interest is highly desirable clinically. To date, while several methods exist to measure global retinal blood flow, they have significant limitations and are not used clinically. For example, ultrasound color Doppler imaging does not have sufficient spatial resolution to measure retinal vessels. It measures velocity and resistive indices in large retrobulbar vessels. Although studies have shown differences between normal and glaucoma groups using ultrasound color Doppler imaging, variability in measurements has limited its potential for clinical diagnosis. Bidirectional laser Doppler velocimetry can measure velocity and flow in individual retinal vessels, but measurement of total retinal blood flow is too time consuming to be practical. Dicon's pulsatile ocular blood flow analyzer can analyze intraocular pressure, but has been shown to be a poor correlate of ocular circulation. Finally, dual-angle Doppler OCT has the limitation of requiring special hardware that is not compatible with existing commercial OCT designs.
As noted above, Doppler OCT, on its own, has difficulty distinguishing the slow flow in small blood vessels from biological motion in extravascular tissue, as well as has difficulty detecting and defining vessel anatomy due to most vessels being nearly perpendicular to the OCT beam. However, Doppler shift can provide valuable quantitative velocity information. Thus, an optimal bulk-noise reduction amplitude-based swept-source angiography method used in combination with Doppler OCT (e.g., via dual scanning, done simultaneously or near simultaneously) could allow for measurement of total retinal blood flow (TRBF), including both vein and artery measurement around the optic disc.